That’s the highest independently measured efficiency for a polymer solar cell, say
researchers from the U.S. Department of Energy’s National Renewable Energy Laboratory
(NREL) and UCLA, who co-authored a report in the Feb. 12 issue of Nature Photonics.

That report, “Tandem polymer solar cells featuring a spectrally matched low-bandgap
polymer,” notes that tandem solar cells by their design can harvest a broader spectrum
of the sun’s rays than single solar cells. But polymer solar cells have lagged because
it’s been difficult finding a suitable low-bandgap polymer.”

In sophisticated tests, the researchers were able to demonstrate highly efficient
single and tandem polymer solar cells featuring a low-bandgap conjugated polymer (PBDTT-DPP:
bandgap, 1.44 eV). When they tested a single-layer device with the polymer it converted
the sun’s rays into electricity at an efficiency of about 6%. When the polymer was
applied to tandem solar cells, the power conversion efficiency reached 8.62%.

The UCLA group recently improved on this result by incorporating a new infrared-absorbing
polymer from Sumitomo Chemical in Japan. NREL measured the power conversion efficiency
at 10.6+/-0.3% under standard terrestrial reporting conditions.

Stacking layers of different materials in a solar cell means multiple bandgaps, each
of which captures a different part of the solar spectrum. The challenge is to achieve
a high current by efficiently using the low-energy portion of the solar spectrum,
and achieving a small energy bandgap – less than 1.5 eV.
The successful test took place in NREL’s Spectrolab X-25 solar simulator, also called
the One-Sun Solar Simulator, which has wide current and voltage ranges.

“Accurately measuring tandem cells is difficult. The NREL simulator provide unparalleled
accuracy by precisely adjusting the spectrum, and did so in a fraction of the time
that other simulators could do the job,” said NREL Principal Engineer Keith Emery.
Each device junction must behave the same under the simulator spectrum as it would
under the reference spectrum. It requires significant adjustment of the simulator
spectrum, normally a very tedious process.

NREL’s One-Sun Solar Simulator was able to turn an ordeal that typically takes all
day into a five-minute task. “We think it’s also more accurate because we can better
adjust the spectrum,” Emery said.

NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for DOE by
the Alliance for Sustainable Energy, LLC.
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